Collaborative relays may be used as “new network elements” or as “transparent network elements”. When used as new network elements it is likely that signaling concepts and/or resource allocation concepts need to at least be partially redefined. In contrast, transparent relay nodes may be (ideally) placed in the wireless network so that they increase the network capacity in such a way that (ideally) the network (or terminals) are unaware of their existence.
One transparent solution may utilize in-band relaying, where at least in part the same frequency is used at the relay input and relay output at essentially the same time. In this case the relay nodes may be added to a wireless system without the need to redesign the base station (transmitter) or the terminals. In such a system the loop interference in amplify and forward relays may be controlled by reducing the relay transmit energy, as perceived at the relay node input. This may be accomplished by separating the receive antennas and transmit antennas from each other (physically or via beamforming). This type of relaying approach is currently being used in DVB-H (test) networks. In addition to DVB-H, this concept is applicable as an add-on feature to prevailing wireless systems, such as WCDMA, or in various OFDM-based systems.
Two-hop solutions have been discussed by A. Wittneben, I. Hammerstroem, and M. Kuhn, “Joint Cooperative Diversity and Scheduling in Low Mobility Wireless Networks,” IEEE Global Telecommunications Conference, Globecom 2004, November 2004; I. Hammerstroem, M. Kuhn, and A. Wittneben, “Channel Adaptive Scheduling for Cooperative Relay Networks,” IEEE Vehicular Technology Conference, VTC Fall 2004, Los Angeles, September 2004 and I. Hammerstroem, M. Kuhn, and A. Wittneben, “Cooperative Diversity by Relay Phase Rotations in Block Fading Environments,” Signal ProcessingAdvances in Wireless Communications, SPAWC 2004, pp. 5, July 2004. In these publications, different time slots are used at the relay for reception and transmission.